MICHAEL CULLEN GEOTECHNICAL LTD

Transcription

1 2282 Seabank Road Courtenay, B.C. V9J 1Y1 Phone (250) for Community of Quatsino Prepared for Regional District Mount Waddington PO Box 729, 2044 McNeill Road Port McNeill, BC, V0N 2R0 November 2014 Community of Quatsino 1 November 2014

2 Executive Summary In May 2014 the Regional District of Mount Waddington were notified by Western Forest Products of a potential land slide risk to the community of Quatsino. Based on initial review it was determined that there is exposure to landslides on Little Creek, Green Creek and Aweisha Creek and that the last major landslides occurred in Michael Cullen Geotechnical (MCG) was tasked with completing a terrain stability assessment and qualitative risk assessment to existing buildings along these creeks. The following table summarizes the results and recommendations where the qualitative risk to existing residential buildings was determined to be greater than low. Building Risk Comments Lot 1 Plan 4153 Very High Recommended action : House Ensure that owners and occupants are made aware of Lot 1 Plan 3505 Very High hazard. Cabin 1 Restrict property use to the months of June through Lot 1 Plan 3505 Cabin 2 Very High August when extreme precipitation events are not expected. Designate lots as a development permit area for hazardous terrain that requires geotechnical assessment prior to any redevelopment on Lot. Parcel A DD House Moderate Existing risk is commonly accepted. No action suggested for existing buildings. Consider designating parcel as a development permit area for hazardous terrain that requires geotechnical assessment prior to All existing buildings on Green Creek fan, see Figure 1 e.g. Lots A, B, C, D, E Plan 2377, Lot C Low to Moderate any redevelopment on Lot. Existing risk is commonly accepted. No action suggested for existing buildings. Consider designating fan as a development permit area for hazardous terrain that requires geotechnical assessment prior to any redevelopment on Lot. Little Creek In November 1989 a landslide initiated on the steep hillside above the creek. The landslide entered Little Creek and became a channelized debris flow. The debris flow travelled down to the foreshore where it destroyed 1 or two houses located on Lot 1 Plan The probability of a similar landslide event occurring is considered high. Sometime after 1990 a new house and shed were constructed on Lot 1 Plan 4153, and 2 cabins were placed on Lot 1 Plan All these structures are located within the deposition zone of the 1989 slide event, and the expected deposition zone of any future large event. All 3 structures are considered to have a specific risk of Very High. It is generally accepted that a risk level of Very High as determined from a qualitative risk analysis is unacceptable and that a more detailed assessment to verify the results and/or mitigative measure should be undertaken. Possible mitigative measure to be considered for the structures on the Little Creek landslide deposition zone include: Community of Quatsino 2 November 2014

3 Restricting property use to the months of June through August 1 when extreme precipitation events are not expected. Placing restrictive covenants or development permit area that do not allow construction to occur on the identified areas of the properties. Following the 1989 landslide some mitigation work was completed on the creek. No written records of this work are available. Based on airphoto interpretation and accounts provided by Quatsino residents the Little Creek channel was excavated through the slide deposit for ~60m from the foreshore. It is understood that in the mid 1990 s Little Creek was relocated to run down the property line between Lot 1 Plan 4153 and Lot 1 Plan Green Creek In November 1989 a landslide initiated on the steep hillside above the creek. The landslide entered the creek and became a channelized debris flow. The debris flow deposition zone from this slide, and other similar slides, is about 700m upstream of the foreshore and existing houses. Flood waters with a high debris and wood load travelled down the remainder of the channel to the foreshore. There was no property damage from the 1989 event or any other known landslide events. The probability of a similar landslide and flood events occurring is considered very high. The specific risk from such an event to existing structures is considered low under existing conditions. A specific risk level of low is generally accepted by society without any mitigation. Green Creek is located on a fan that is presently considered stable. It is conceivable that a debris flow could block the channel and cause avulsion resulting in water affecting an existing structure on the fan. The potential for this occurring is considered moderate (e.g. an event is not expected under normal conditions but may occur under adverse conditions), the vulnerability from such an event is considered low to moderate and the specific risk is low to moderate A specific risk level of low to moderate is generally accepted by society without any mitigation. Following the 1989 landslide some mitigation work was completed on the creek. No written records of this work are available. Based on airphoto interpretation, field observations and accounts provided by Quatsino residents the Green Creek channel was excavated for ~450m from the foreshore. The excavated material was used to construct a raised berm along the sides of the channel. This mitigation work is expected to adequately convey flood waters. Alders that have now fallen into the channel pose an obstruction threat that could ultimately result in channel erosion, avulsion, and damage to the bridge on the road that runs through the community. We recommend that the woody debris accumulating within the constructed channel be periodically removed. 1 Quatsino resident David Addison reports that the house on Lot 1 Plan 4153 is presently used only intermittently throughout the year, and that only one of the cabins on Lot 1 Plan 3505 is used as a residence and that it is presently only used occasionally during summer months. Community of Quatsino 3 November 2014

4 Aweisha Creek In November 1989 several landslides initiated from logging roads above the creek. The landslides entered the creek and became a channelized debris flow or debris flood. The debris deposition zone from this slide (and other similar slides) is about 1000m upstream of the foreshore and existing houses. Flood waters with a high debris and wood load travelled down the remainder of the channel to the foreshore. There was no property damage from the 1989 event or any other known landslides. The road was washed out presumably due to the culverts becoming blocked or overwhelmed. The probability of a similar landslide and flood events occurring is considered high. The specific risk from such an event to existing structures is considered low under existing conditions. A specific risk level of low is generally accepted by society without any mitigation. Following the 1989 landslide some mitigation work was completed on the creek. No written records of this work are available. Based on airphoto interpretation, field observations and accounts provided by Quatsino residents the channel was excavated for ~250m from the foreshore. The excavated material was used to construct a raised berm along the sides of the channel. This mitigation work is expected to adequately convey flood waters. Alders that have fallen into the channel now pose an obstruction threat that could ultimately result in channel erosion, avulsion, and damage to the culverts on the road that runs through the community. We recommend that the woody debris accumulating within the constructed channel be periodically removed. Community of Quatsino 4 November 2014

5 Table of Contents Executive Summary... 2 Introduction... 6 Historic and Background Information... 6 Geomorphology Information... 6 Historic Records of 1989 Slide... 7 Western Forest Products Records... 9 Historic Imagery Precipitation and Landslides Seismic Hazard and Landslides Field Observations and Discussion Little Creek Green Creek Aweisha Creek Qualitative Landslide Risk Analysis Risk Acceptance and Mitigation Conclusions and Recommendations Study Limitations and Conditions Closure Figure 1: Google Earth Image showing subject creek/slide paths Figure 2: 1992 Airphoto Figure 3: 2012 orthophoto Figure 4: Google Earth Image and Profile along Little Creek Figure 5: Google Earth Image and Profile along Green Creek Figure 6: Google Earth Image and Profile along Aweisha Creek Photos Appendix 1: Excerpts from Geomorphology of Vancouver Island Appendix 2: Seismic Hazard Community of Quatsino 5 November 2014

6 Introduction In May 2014 the Regional District of Mount Waddington was notified by Western Forest Products of a potential land slide risk to the community of Quatsino. Two landslide paths were identified as a potential threat to residential structures in the community. Michael Cullen Geotechnical (MCG) was awarded a contract to complete a terrain stability assessment and a qualitative risk assessment to determine the exposure of existing residential buildings to the 2 landslides. Funding for the project was provided by EMBC. In the course of the initial background information review it was determined that 3 creeks had seen landslides on November during a storm event which delivered 161mm of precipitation in 24 hours. Our scope of the work was expanded to include the 3 rd landslide. The creeks associated with the slides are called Little, Green and Aweisha; they are shown on Figure 1. During the field work it was discovered that some mitigative measures had been completed on the 3 creeks in Our scope of work was expanded to also include an assessment of the functionality of these mitigative measures. Historic and Background Information Geomorphology Information Overview geomorphology mapping prepared by R Guthrie 2 provide initial information of surficial geology, bedrock geology, potential for mass wasting (landslides), and potential for gullying. Excerpts from the maps are included in Appendix 1. The following summarizes the information from these maps: Bedrock for the project site is mapped as being Queen Charlotte Group undivided sedimentary rocks. Conglomerate was identified throughout the study area. The surficial soils in the project area are mapped as fluvial sediments in the lower reaches of Green and Aweisha Creek and shallow colluvium over bedrock in the upper reaches. Glacial fluvial sediments are mapped along Little Creek. This is consistent with our observations in the field. A large fluvial fan is present on the lower reaches of Green Creek as indicated on Figure 1. Thick glacial deposits were noted along Little Creek. The steep side slopes above Little, Green, and Aweisha creeks are mapped as having a mass wasting potential defined by slopes greater than 60%. The areas fall within Zone I 2 Guthrie, R, 2005 Geomorphology of Vancouver Island BC Ministry of Environment Community of Quatsino 6 November 2014

7 Wet West Coast. This is consistent with field observations. Rock falls, rock slides, open slope slides and debris flows are occurring in these from slope steeper than about 65%. The slopes along Little, Green, and Aweisha creeks are mapped as having some gullying potential. This is consistent with field observations. Gullying was noted in the field. In Green and Aweisha Creeks gully depth was limited to several meters on account of shallow bedrock. Historic Records of 1989 Slide Historic records of landslides, compiled by D Septer 3, provide the following account of the 1989 slide: Source: The Vancouver Sun, November 10, 13 and 22, 1989; December 15, 1989; Times Colonist, November 10 and 21, 1989; November 10, 1990; The Province, November 12, and 14, 1989; The Chilliwack Progress, December 5, 1990; Miles et al. 1979; Rice et al. 1992; Di Cenzo 1990; Andrews 1993; Evans, unpublished data; Nesbitt-Porter 1989; Ministry of Environment files. Details: On November 9, torrential rains and gale-force winds occurred throughout British Columbia. The rainfall broke many records.hardest hit were the Chilliwack area, Port Alice and Rivers Inlet.On November 9, a debris torrent occurred at a creek near Quatsino on the west coast of Vancouver Island (Andrews 1993). The mudslide smashed a home off is foundations onto the beach below. The damage was estimated at $80,000. The roof was the only part not destroyed. A debris avalanche on the west slope triggered the torrent that occurred on Little Creek. This avalanche comprised a failure of the thin veneer of soil and organics together with the tree cover sliding over the underlying conglomerate bedrock. The avalanche was located on the north boundary of a small block that apparently was cleared (clearcut) in the 1950s. It extended up to elevation 300 m but was a little more than m wide. The total of debris, including trees, deposited on the fan above sea level was estimated at 20,000-30,000 m3. Discharges as high as m 3 /s (10-20 times the estimated 200-year flood) could have occurred upstream of the fan during the event. The debris torrent deposited gravel on the alluvial fan and destroyed two houses. A third house, located on the left bank, was not destroyed but deemed unliveable by the District 4....The prime cause of the torrents on Little and Green creeks appeared to have been the lengthy period culminating in high precipitation. A debris torrent event also occurred on Green Creek. This torrent terminated at a bend in the creek. It resulted in the deposition of a substantial quantity of wood debris (logs) as well as sand, gravel and boulders up to 0.5 m. Some floodwater overflowed the creek banks, flowing down a water supply ditch excavated in October 1989 and through a cleared area adjacent to a house. *6) The channel of the creek was completely infilled with gravel. Also a community water supply was damaged This account of 2 houses destroyed and a third house deemed unliveable could not be verified. It is not consistent with other accounts or resident statements. Community of Quatsino 7 November 2014

8 6) Similar debris torrents have previously occurred on Green Creek. The 1980 aerial photographs, taken prior to the recent logging on privately owned land, showed distinct evidence of a recent torrent, which terminated at approximately the same location. Two debris avalanches and a rockslide area were observed in the headwaters of the creek. A letter from the Province of British Columbia dated May 2, 1990 states that: The Provincial Emergency Program Agrees to provide a maximum of $100,000 funding through the Disaster Financial Assistance Program for debris removal on Aweisha Creek, Green Creek and Creek which terminates on Plan 3505, Plan 4153 and DD The Project will include removal of debris on the three creeks to re-establish a free flowing creek and installation of a waterline The work described above only became known during our field work. No engineering or construction records of the work completed could be located from the agencies contacted (RDMW, MFLNRO, EMBC, MOE), or through written references to the event. Allan Johnson, a resident of Quatsino at the time of the 1989 landslides provided the following account of the event and subsequent mitigative work: Little Creek: the landslide pushed a house located on Lot 1 Plan 4153 off its foundations and into the bay. The house located on Lot A DD was not affected however some landslide debris was located within a few meters of the house. After the slide event the location of the creek had shifted to the east, an excavator was used to create a deeper channel over the first ~60m of the creek. Subsequently the creek was realigned to run down the property line between Lot 1 Plan 4153 and Lot 1 Plan Several years later a new house was built on Lot 1 Plan 4153 and 2 cabins were dragged up from the beach onto the adjacent property to the west. Green Creek: there was no damage to any structures or the bridge from the 1989 event. Most of the debris was deposited in the channel upstream of the road (~700m). Some water was noted flowing overland approximately 50m upslope of the road. Green Creek followed a very shallow channel prior to 1989 event. Following the event an excavator was used to create a deeper channel extending from the foreshore to ~450m up the creek. A staging area was developed on the west side of the creek. A small gravel quarry also operated on the west side of the creek which is the cause of the disturbance and anomalous gravel deposits in the woods along the west side of the creek. Aweisha Creek: there was no damage to any structures. The culverts and road were washed out. People were able to cross the creek during the flood. Most of the debris was deposited in the channel upstream of the road (~1000m). Following the event new culverts were installed and an excavator was used to create a deeper channel extending from the foreshore to ~200m up the creek. Community of Quatsino 8 November 2014

9 Western Forest Products Records Western Forest Products hold tenure that includes the upper reaches of Green Creek and the areas surrounding the Project site. WFP personnel 5 provided the following assessments: The area is in the High Regional Landslide Frequency zone as per WFP s TRMS maps, which means that the average over the region is greater than 5 landslides/100 ha logged on steep terrain. For example, the Lewis watershed bordering this gully system had 8.4 landslides/100 ha logged steep terrain in post Code blocks as of the 2006 inventory. The image below shows the extent of steep terrain (orange) in the vicinity of the gully. Steep includes Class 4 and Class 5 terrain stability polygons plus slopes steeper than 60% (generated by TRIM DEM) that fall outside those polygons. We include the >60% slopes as to help normalize different standards of terrains stability mapping, so as to compare landslide occurrence across WFP s tenures. Red dots are landslide initiation points. The yellow area on the image is WFP s tenures. Landslide information may not be complete in the white areas outside WFP s tenure. 1 is Little Creek, 2 is Green Creek 5 Personal communications with Dave Mogensen (Senior Timberlands Forester), and Glynnis Horel (Geotechnical Consultant to WFP) Community of Quatsino 9 November 2014

10 The image below delineates the forest cover polygons and displays year of stand establishment of leading species. Blocks logged after 2005 are not identified. The following observations/interpretations are based on our review of the historic images: Slide #1(Little Creek) 1965 and 1992 photos show that a logged area exists outside against the TFL 6 boundary but outside of TFL 6, near the upper slide region, and is indicated as regenerated in The 1965 photo shows the logging below the slide #1 area is greened up, and that no slide yet exists. The 1992 photo shows a very fresh slide, and well developed regeneration in the logged area. The slide originates on slope above the area harvested. The regeneration on the harvested area appears to have been >30 years old. Slide #2 (Green Creek) this area in the 1992 photo, appears to consist of 2 initiation points in unharvested timber with run out to the waterfront. There are also older paths and initiation points visible in the gully complex. WFP s harvesting nearest to upper slide #2 area was in Slide 3 (Aweisha Creek) exists in the 1992 photo to the east of the Slide #2 area, and has a separate run out path to a delta on the waterfront -- east of the slide #2 fan. The initiation point appears to be in harvesting conducted outside TFL 6, but the initiation point is just off the edge of the photo. Community of Quatsino 10 November 2014

11 Historic Imagery The following historic imagery was reviewed: Airphotos: o A3294 (1932) # 84, o BC5137 (1965) #116 and 117 o BC5483 (1972) #114 and 115 o BC80070 (1980) #21, 21, 25, 26 o 30BC92029 (1992) #47, 48, 46, 25, 26 o 30BCC96168 (1996) #24, 25, 39, 40 Orthophoto (2012) courtesy of Western Forest Products Google Earth Image (2006) Based on our interpretation of the imagery we conclude the following: Landslides are obvious on Green and Aweisha Creek back to The initiation point of all identified landslides from 1932 to 2014 are shown on Figure 1. Shoreline deltas (indicative of creeks with high bed load or sediment load) are obvious on Green and Aweisha Creek back to A large fluvial fan is evident on Green Creek, see Figure 1. There is no evidence of channel avulsion leading us to conclude that the fan is presently stable. The deposition zone for most landslides on Green Creek is 700 to 1000m above the foreshore, see Figure 1 The deposition zone for most landslides on Aweisha Creek is 1000 to 1300m above the foreshore, see Figure 1. There is evidence that two slides occurred on the upper west side of the Green Creek valley that triggered the 1989 event, see Figure 2. Slides in similar locations are evident on the 1932 airphotos indicating that the slopes in this area are naturally unstable. Aweisha Creek valley was extensively logged in the 1980 s. There is evidence of numerous open slope and road related landslides following harvesting. There is evidence that three and possibly 5 slides occurred off forest roads around Aweisha Creek that triggered the 1989 event. The only landslide identified on Little Creek is the 1989 event. There is no evidence of foreshore delta except following the 1989 event. The area around Little Creek was logged in the early 1960 s. The 1989 landslide initiated just above the upper falling boundary. On all post 1990 images there is considerable disturbance on the lower reaches of Little, Green and Aweisha Creeks, see Figure 2 and 3. This disturbance was originally interpreted to be the result of the 1989 landslide events but was subsequently determined to be partially due to channel repair work completed in 1990 to mitigate future hazard. The initial interpretation lead to a belief that the 1989 events were much larger and posed a much greater hazard than reality. Community of Quatsino 11 November 2014

12 Precipitation and Landslides Precipitation is the single greatest trigger of landslide events in coastal BC. Extreme precipitation, possibly coupled with high winds causing tree windthrow, was the cited trigger for the 1989 landslides. Quatsino has been recording climate information since Precipitation norms and extremes for Quatsino are provided in Table 1. Unfortunately Environment Canada reports that the data is not adequate to develop reliable rainfall intensity duration prediction curves. Based on the available data extreme rainfall events may occur in any month except June, July and August. Table 2 presents a summary of extreme rainfall events for Quatsino as determined from Environment Canada data 6. The available information suggests that large landslide events may be triggered in Green and Aweisha Creek by 24 hour precipitation events exceeding ~129mm coupled with high antecedent precipitation. The return period for such events in Quatsino is approximately 1:20 years. The only identified slide on Little Creek occurred in 1989 following 161mm precipitation in 24 hours and 171mm 7 day antecedent rain. We suspect that windthrow along a falling boundary was also a significant trigger in this event. There is insufficient data to establish a return period for such events (which include anthropogenic activity); based on the available data we conservatively estimate the return period to be less than 1:100 years. Table 1: Precipitation Norms and Extremes for Quatsino 6 Community of Quatsino 12 November 2014

13 Table 2: High and Extreme Rainfall Events for Quatsino Date 24 hour 7 day Antecedent Comments precipitation precipitation November December December November 3, September 18, May 29, Suspected trigger for Green Creek slide seen on 1932 airphoto. October 19, April 6, April 11, November 12, Suspected trigger for Green Creek slide seen on 1980 airphoto. December 9, December 10, November 8, Trigger for Little, Green and Aweisha Creek slides November 7, September 24, Trigger for Green Creek slide. Also trigger for many other slides on Northern Vancouver Island Seismic Hazard and Landslides Earthquakes are often triggers of landslides; however, there is no evidence of any recent earthquake triggered landslides in the vicinity of the assessed creeks. The seismic hazard for Quatsino was determined from natural resources Canada 7. The results are presented in Appendix 2. The site is considered to have a relatively high seismic hazard with a predicted peak ground acceleration of 0.246g for the 1:2475 earthquake. This level of earthquake could conceivably trigger a landslide. 1:2475 is the equivalent of an annual probability of which is considered a low to very low probability of occurring and therefore the probability of a landslide triggered by an earthquake is also considered low. 7 Community of Quatsino 13 November 2014

14 Field Observations and Discussion Field inspections were completed by Michael Cullen of MCG and Patrick Donaghy of RDMW on July 3 and The field work included low level helicopter reconnaissance of the entire project area, walking Green Creek from the cut block in the upper reaches of the valley to the foreshore, walking the lower reaches of Little Creek, Aweisha Creek, and the Green Creek fan. Little Creek Little Creek has a watershed area less than 0.3 km 2. Much of the watershed was logged in the 1960 s. The creek has a wetted flow area of 0.2 to 0.5 m 2 and is seasonally dry. Creek gradient ranges from less than 5% in the lower reaches up to 30% in upper reaches, see Figures 3 and 4. Table 3 summarizes our observations. Table 3: Observations along Little Creek Reach Channel Side Slopes Comments approx. m grad. % material width m angle % material height m 0 to 60 <5 fluvial fluvial 1.5 Excavated channel 60 to 200 <10 fluvial colluvium 12 Minor accumulation of gravel and cobble wedges and small woody debris in channel. 200 to bedrock 8 85 glacial 40 There is evidence of instability on the side slopes including pistol butt trees, small historic slumps. There is evidence of ongoing small slumps along creek channel due to over steepened slopes and undercutting. Alders extend 15m up slopes presumably on account of 1989 slide. Occasional small debris jams present. The 1989 slide occurred on the hillside above the west side of the creek, see Figure 2 and 4. The headscarp of the slide is approximately 20m above the upper falling boundary of a cut block logged in the 1960 s. Slope gradient at the headscarp is approximately 70%, slope material consists of a veneer of colluvium over bedrock. The slide path above Little Creek is about 400m long, 15 to 30m wide, and 1 to 2m deep. The slide material entered Little Creek where it became a debris flow that travelled to and deposited on the foreshore. The extent of the deposition zone is shown on Photo 1. We estimate that the volume of material deposited is in excess of 20,000 m 3. Historic records suggest that 1 and possibly 2 houses were destroyed by the slide. Apart from the 1989 event there is no other historic evidence of landslides and debris flows in Little Creek. However, the slopes on the west side of Little Creek are classified Terrain Class 4 and 5 and there is no information that distinguishes these slope from the slopes in the adjacent Green Creek that experience a high rate of landslides. Given these conditions and expected extreme precipitation return period of less than 1:100 years we consider that the potential for another landslide above Little Creek is high. Community of Quatsino 14 November 2014

15 Little Creek presently only has minor debris jam accumulations. However, the side slopes along Little Creek are considered only marginally stable. Over time small side slope failures are expected to occur that will introduce material to the creek channel. Given these conditions and the high landslide potential discussed above we consider that the probability of another debris flow on Little Creek is high. Sometime after 1990 a new house and shed were constructed on Lot 1 Plan 4153, and 2 cabins were placed on Lot 1 Plan As shown on Photo 1 these structures are within the deposition zone of the 1989 slide event. If a similar size landslide or debris flows occurs these structures will most likely be severely impacted. The excavated ditch that extends ~60m from the foreshore is only expected to manage typical creek water flow; the ditch would be overwhelmed in the event of a landslide or a debris flood. Green Creek Green Creek has a watershed area of about 0.7 km 2. The creek has wetted flow area of 0.6 to 0.8m 2. Creek gradient is less than 10% over the lower 1000m with several reaches less than 5%, see Figure 3 and Figure 5. The gradient increases to between 15 and 40% in the upper reaches. Table 4 summarizes our observations. There are residential buildings located close to Green Creek along the shore line, see Figure 1 and Photo 5. All buildings are located greater than 60m from the channel. It is understood that during the 1989 and the 2010 events creek flow remained in the channel and there was no damage to property or bridge. The 1989 debris flow appears to have initiated from an active landslide gully on the hillside approximately 400m above the west side of the creek, see Figure 2, Figure 5 and Photo 4. Slope gradient at the headscarp of the slide is approximately 80%. Most of the debris from the 1989 landslide (as well as others landslides noted on airphotos and in the field) deposit out between 700m and 1000m upstream of the houses and bridge, see Figure 1. Flow beyond this deposition zone is expected to be primarily flood water with a high debris load; it is not expected to be a debris flow. The excavated channel (discussed below) is expected to be able to handle the flow. There was no property damage from the 1989 event or any other recorded landslides on the Creek. Community of Quatsino 15 November 2014

16 Table 4: Observations along Green Creek Reach Channel Side Slopes Comments approx. m grad. % material width m angle % material height 0 to 117 <5 fluvial 4 to 6 80 fluvial 1.5 Excavated channel. Appears that some rip rap has been placed on east side 117 to fluvial Bridge 125 to fluvial to 80 fluvial 2.5 to 4 Excavated channel. See Photo 2 Height includes a constructed berm up to 1.2m high and 5m wide. River scour is resulting in small failures of excavated side slopes. Some alders are toppling into channel. 400 to fluvial to 120 fluvial 2.5 to 4 Excavated channel. Height includes a constructed berm to 1m high. River scour is resulting in small failures of excavated side slopes. Alders along the side slopes are toppling into creek and present potential obstruction. 450 to 676 <10 fluvial to 100 fluvial 4 to 6 Natural channel Mature confers and alder on sides. 676 to fluvial 8 to to 80 fluvial 4 Slide deposition zone. This is the suspected primary deposition zone for all previous slides including for 1989 event m debris deposit at 800m is assumed to be from September 2010 slide event see Photo to to 50 bedrock 10 Inner 80 to 120 Outer 70 to 90 Blanket of Colluvium and residual soil over rock Inner 50 Outer >100 Channel incised 1 to 5m into bedrock. Stepped channel. Occasional small debris jams. Inner and outer slopes above channel show evidence of numerous open slope slides up to 3m deep, 100m long, and 30m wide. Slides typically remove colluvium and residual soil down to bedrock. On inner slopes pistol butt trees, tension cracks, and slumps are common. Evidence of rockfall and rockslides from bluffs in higher reaches of creek. Evidence of active gullies; Photo 4 shows gully that 1989 event occurred on, incidence angle of gullies to main channel is between 45 and 90 degrees. Following the 1989 landslide some mitigation work was completed on the creek. No written records of this work could be located. Based on airphoto interpretation, field observations and accounts provided by Quatsino residents the channel was excavated for ~450m from the foreshore. The excavated material was used to construct a raised berm along the sides of the channel. The excavated channel above the bridge has a cross section area greater than 15m 2. The area under the bridge has a cross section area of about 10m 2. There are alders growing along and falling into the excavated channel. These trees pose an obstruction threat that could ultimately result in channel erosion, avulsion, and damage to the bridge. We recommend that the woody debris accumulating within the constructed channel be periodically removed. There is a large fan on the creek, see Figure 1. There is only one channel incised/excavated into the fan. There are reports of some overland flow on the fan to the west of the creek during the Community of Quatsino 16 November 2014

17 1989 landslide event that was attributed to water entering a channel cut in the side of the creek for a water supply pipe line. We did not find any evidence of recent channel avulsion or active secondary channels on the fan during our field work. Evidence of paleo channels was noted on the upper western side of the fan. The potential for future channel avulsion cannot be ruled out; a debris flow could conceivably block the channel and cause the water to take a new path. However; on account of the relatively deep (4m) channel in the primary deposition zone, and presence of mature timber on the fan, we consider that the probability of such an event to occur and reach the existing houses to be low to moderate. Aweisha Creek Aweisha Creek has a watershed area of about 0.9 km 2. The creek has wetted flow area of ~0.8m 2. Creek gradient is typically less than 5% over the lower 1000m, see Figure 3 and Figure 6. Gradient is up to 50% in the upper reaches. Table 4 summarizes our observations. Table 5: Observations along Aweisha Creek Reach Channel Side Slopes Comments approx. m grad. % material width m angle % material height m 0 to 40 2 fluvial 6 60 fluvial 1.5 Excavated channel. Appears that some rip rap has been placed on east side 40 to 55 Road with a 1500mm and 1200mm CMP 55 to 90 3 fluvial to 80 fluvial 2.5 to 4 Headpond. See Photo 7. Headpond is being maintained Height includes a constructed berm up to 1m high and 3m wide. 90 to 250 <5 fluvial to to to 8 fluvial to 120 fluvial 2 to 4 Excavated channel. Height includes a constructed berm up to 1m River scour is resulting in small failures of excavated side slopes. Alders along the side slopes are toppling into creek and present potential obstruction. Occasional small debris jams forming. fluvial 2.5 to 6 Natural channel Mostly alders on sides. Alders along the side slopes are toppling into creek and present potential obstruction. Occasional small debris jams forming 500 to fluvial Natural channel 900 to 1200 <10 Slide deposition zone. This is the suspected primary deposition zone for all previous slides including 1989 event to to 50 Bedrock? 5 to to 100 Colluvium and residual soil over rock >100 Evidence of numerous older forest harvesting related slides (both open slope and road). Occasional slides associated with old roads continue to occur. Community of Quatsino 17 November 2014

18 There are residential buildings located close to the creek along the shore line, as well as 300m above the shoreline see Figure 1 and Photo 6. All buildings are located more than 50m from the creek. Much of the watershed was logged in the 1980 s. Post logging there was a significant increase in the rate of landslides in the watershed. The rate of landslides appears to have tapered off in recent years; however, there is still evidence of relatively fresh landslides that initiated off old logging roads. The 1989 slide event appears to be related to between 2 and 5 failures initiating from logging roads, see Figure 2 and Photo 8. Most of the debris from the 1989 landslide (as well as other landslides noted on airphotos) deposit out between 1000m and 1300m upstream of the foreshore, see Figure 1. Flow beyond this deposition zone is expected to be primarily flood water with a high debris load; it is not expected to be a debris flow. The excavated channel (discussed below) is expected to be able to handle the flow. There is not reported property damage from any landslides. The road culvert was blown out during the 1989 event. Following the 1989 landslide some mitigation work was completed on the creek. No written records of this work could be located. Based on airphoto interpretation, field observations and accounts provided by Quatsino residents the channel was excavated for ~250m from the foreshore. The excavated material was used to construct a raised berm along the sides of the channel. There are alders growing along and falling into the excavated channel. These toppled trees pose an obstruction threat that could ultimately result in channel erosion, avulsion, and plugging the culverts. We recommend that the channel be cleared of fallen wood on a regular basis. Community of Quatsino 18 November 2014

19 Qualitative Landslide Risk Analysis Qualitative risk assessments are commonly used to establish if a potential risk exists and as a screening tool to determine if further investigations and/or analysis, and/or mitigative measures should be implemented. Landslide risk is defined as the combination of the probability of occurrence and consequence of occurrence. Mathematically this can be written as: Risk = P(H) x P(S:H) x P(T:S) x V x E equ. 1 where: P(H) is the probability of occurrence of a specific hazardous landslide P(S:H) is the probability that there will be a spatial effect given that a specific hazardous landslide occurs P(T:S) is the temporal probability or likelihood that habitants are present at the time of the slide V is the vulnerability of the building or persons. E is the elements at risk. E.g. number of habitants exposed. For screening level qualitative risk assessments equ. 1 is simplified to: Risk = P(HA) x V equ. 2 Where P(HA) is the partial risk or probability of occurrence of a specific hazardous landslide and the probability of it reaching or otherwise affecting the site occupied by a specific element. P(HA) = P(H) x P(S:H) Qualitative risk analysis uses relative ratings for each of the values in equation 2. Tables 5, 6 and 8 provide the definitions used in this assessment. Tables 7 and 9 show how the values are combined to arrive at a specific risk value. Table 10 provides a summary of the risk for existing residential buildings in the vicinity of Little Creek, Green Creek and Aweisha Creek where the risk was assessed to be greater than low. Community of Quatsino 19 November 2014

20 Table 5: Probability of occurrence of a specific hazardous landslide (P(H) ) Rating Annual Probability of Occurrence Probability (%) of an event in 50 years. Very Low Less than 0.4% Less than 1:2475 Low 0.4 to 0.2% 1:2475 to 1:500 Moderate 0.2 to 1% 1:500 to 1:100 High 1 to 5 1:100 to 1:20 Very High Greater than 5% Greater than 1:20 Table 6: Less than 2% 2 to 10% 10 to 39% 39 to 92% Greater than 92% Probability of specific hazardous landslide event affecting a house (P(S:H)) Rating Low Likelihood of an event is very remote Moderate An event is not expected under normal conditions but may occur under adverse conditions. High Criteria No evidence of past landslide activity House not located on a fan House more than 20m from edge of creek No evidence of past landslide activity House located on a fan House located closer than 20m to creek Evidence of previous landslides Table 7: Probability of Occurrence P(H) Matrix to Determine Partial Risk P(HA) from (P(H) ) and (P(S:H)) Probability that there will be a special effect (P(S:H)) Low Moderate High Very Low Very Low Very Low Low Low Very Low Low Moderate Moderate Low Moderate High High Moderate High Very High Very High High Very High Very High Table 8: Rating Low Moderate High Definition of Vulnerability (V(L:T)) Criteria No significant structural damage is expected e.g. flooding damage only Some structural damage may occur that is repairable e.g. damage from debris flood impact Significant structural damage occurs that would be life threatening to persons in building e.g. debris flow or landslide impact Table 9: Partial Risk P(HA) Matrix to Determine Specific Risk R(S) from P(HA) and V(L:T) Vulnerability V (L:T) Low Moderate High Very Low Very Low Very Low Low Low Very Low Low Moderate Moderate Low Moderate High High Moderate High Very High Very High High Very High Very High Community of Quatsino 20 November 2014

21 Table 10: Existing residential buildings with landslide risk greater than low Property P(H) P(S:H) P(HA) V(L:T) Specific Risk R(S) Lot 1 Plan 4153 High High Very High High Very High House Lot 1 Plan 3505 High High Very High High Very High Cabin 1 Lot 1 Plan 3505 High High Very High High Very High Cabin 2 Parcel A High Low Moderate Moderate Moderate DD House Existing Buildings on Green Creek Fan exposed to potential flooding, see Figure 1. e.g. Lots A, B, C, D, E Plan 2377 Lot C Moderate Moderate Moderate Low to Moderate Low to Moderate Risk Acceptance and Mitigation The Regional District of Mount Waddington has not established acceptable risk levels for landslides affecting either new or existing structures. For new development in British Columbia Section 56 of Community Charter and section 910 of the Local Government Act and Land Title Act require that with respect to landslides the land may be used safely for the use intended. However, the term safely is not defined. Commonly used definitions for safe in British Columbia include the following: Less than a 1 in 475 probability of damaging event under static loading conditions. Less than a 1 in 2475 probability of damaging event under seismic loading. 1:100,000 risk to life. Hazard Acceptability Thresholds for Development Approvals by Local Governments 8 The City of North Vancouver and the District of North Vancouver have recently completed qualitative risk assessments and have adopted the following strategies; For existing buildings with a qualitative risk rating of high, very high or extreme complete detailed quantitative risk assessments and/or implement mitigative measures. Houses rated very high or extreme warrant immediate attention. For existing buildings with a qualitative risk rating of low or moderate no specific mitigative measures are recommended. Continued monitoring is recommended. Based on the above criteria we recommend that additional investigations and/or detailed quantitative risk analysis, and/or mitigative measures be completed for the house on Lot 1 Plan 4153 and Cabins on Lot 1 Plan Possible mitigative measures to consider include: Restricting property use to the months of June through August when extreme precipitation events are not expected. It is understood to some extent this may already 8 Community of Quatsino 21 November 2014

22 be occurring as Quatsino residents report that the house on Lot 1 Plan 4153 is presently used only intermittently, and that only one of the cabins on Lot 1 Plan 3505 is occasionally used during summer months. Placing restrictive covenants or development permit requirements that do not allow construction to occur on the identified areas of the properties. Conclusions and Recommendations The terrain stability assessment has determined that there is a high likelihood that landslides, including debris flows, will occur on Little Creek, Green Creek and Aweisha Creek. There is evidence of both natural and forestry related landslides on the 3 creeks. The expected landslide deposition zone is at the foreshore on Little Creek, 700m above the shoreline on Green Creek, and 1000m above the shore on Aweisha Creek. There is stable fan on Green Creek. The following table summarizes the results and recommendations from the qualitative risk analysis where the risk to existing residential structures was determined to be greater than low. Building Risk Comments Lot 1 Plan 4153 Very High Recommended action : House Ensure that owners and occupants are made aware of Lot 1 Plan 3505 Very High hazard. Cabin 1 Restrict property use to the months of June through Lot 1 Plan 3505 Cabin 2 Very High August when extreme precipitation events are not expected. Designate lots as a development permit area for hazardous terrain that requires geotechnical assessment prior to any redevelopment on Lot. Parcel A DD House Moderate Existing risk is commonly accepted. No action suggested for existing buildings. Consider designating parcel as a development permit area for hazardous terrain that requires geotechnical assessment prior to All existing buildings on Green Creek fan, see Figure 1 e.g. Lots A, B, C, D, E Plan 2377, Lot C Low to Moderate any redevelopment on Lot. Existing risk is commonly accepted. No action suggested for existing buildings. Consider designating fan as a development permit area for hazardous terrain that requires geotechnical assessment prior to any redevelopment on Lot. Based on generally accepted practices we recommend that additional investigations, and/or detailed quantitative risk analysis, and/or mitigation be completed on all buildings where the specific risk is greater than moderate. Community of Quatsino 22 November 2014

23 Study Limitations and Conditions Michael Cullen Geotechnical Ltd (MCG) prepared this report for the use of Regional District of Mount Waddington (the Client). MCG does not accept liability for any damages suffered where a third party uses this report, or where it is used for purposes other than intended. This document including all text, data, figures, drawings and other articles prepared by MCG are considered its professional work product and shall remain the copyright property of MCG. The Client may make copies of the document in such quantities as are reasonably necessary for those parties affected by or conducting business specifically related to the subject of this document, or in support of regulatory requirements. This report has been prepared in a manner consistent with the level of care and skill ordinarily exercised by engineers currently practicing under similar conditions in the jurisdiction in which the services are provided, subject to the time limits, directives of the client, and physical constraints applicable to this document. No warrantee express or implied is made. This written report is of a summary nature and is not intended to stand alone without reference to the instructions given to us by the Client, communications between us and the Client, and to any other documents relative to the subject project, which in aggregate form the whole report. In order to properly understand the recommendations and opinions expressed herein, reference must be made to the whole of the Report. The conclusions and recommendations in this report are based on surface observations only in select locations within the project area. No subsurface investigations or materials testing were completed. The field investigation cannot practically cover the entire area and will only identify conditions at the point and time of sampling. Geological and hydrological conditions can vary significantly over a very short distance, or depth, and may also change with time such that the investigation program may fail to detect or identify all subsurface conditions; actual conditions encountered may vary significantly from observation locations. If conditions other than those presented in this report are encountered MCG should be contacted to evaluate the impact of the changed conditions on the conclusions and recommendations presented in this report. The conclusions and recommendations in this report are based on information made available to MCG at the time the report was prepared. We have relied in good faith upon representations, information and instructions provided by the Client and others concerning the site. MCG is not liable for any deficiency, misstatement or inaccuracy contained in the Report as a result of misstatements, errors, omissions, or misrepresentations, of others. The information, interpretations and conclusions in the Report are based on our interpretation of conditions revealed through a limited assessment, conducted within a defined scope of services. MCG cannot accept responsibility for independent conclusions, interpretations, interpolations and/or decisions of the Client, or others, which may be based on information contained in the Report. Community of Quatsino 23 November 2014

24 Closure We trust that this report satisfies your present requirements. Should you have any questions, please do not hesitate to contact us. The opportunity to be of service to you is appreciated. Sincerely Michael Cullen Geotechnical Ltd. per Michael Cullen, P.Eng. Community of Quatsino 24 November 2014

25 Figure 1: Google Earth Image showing subject creek/slide paths and identified landslide initiation locations (purple circles). Green Creek Historic fluvial fan Little Creek and 1989 slide path path of 1989 slide Green Creek Aweisha Creek Deposition Zones for Landslides Community of Quatsino 25 November 2014

26 Figure 2: 1992 Airphoto. The circled areas of extensive disturbance were originally considered to be the result of the 1989 landslide events but were subsequently determined to be partially due to channel repair work completed after the events. Arrows point to suspected slide initiation points Community of Quatsino 26 November 2014

27 Figure 3: 2012 orthophoto with 20m TRIM contours overlaid. Courtesy of Western Forest Products. Extent of disturbance from 1989 landslide event and subsequent mitigative measures can still be seen; areas are mapped out in red for Little Creek (1) and Green Creek (2). Community of Quatsino 27 November 2014

28 Figure 4: Google Earth Image and Profile along Little Creek Headscarp of 1989 slide Community of Quatsino 28 November 2014

29 Figure 5: Google Earth Image and Profile along Green Creek One of the initiation points of 1989 slide event Community of Quatsino 29 November 2014

30 Figure 6: Google Earth Image and Profile along Aweisha Creek One of the initiation points of 1989 slide event off old road Community of Quatsino 30 November 2014

31 MICHAEL CULLEN GEOTECHNICAL LTD Photos Photo 1: Little Creek Development on Slide deposition zone 2 Cabins moved onto Lot 1 Plan 3505 after the 1989 slide Approximate extent of 1989 landslide deposition zone New house and shed constructed on Lot 1 Plan 4153 after 1989 slide House on Parcel A DD was not impacted by 1989 slide Present location of Little Creek in an excavated channel 1.5m deep and 2.5m wide Photo 2: Excavated channel with side berm on Green Creek. Community of Quatsino 31 November 2014

32 MICHAEL CULLEN GEOTECHNICAL LTD Photo 3: Green Creek 2010 slide deposition zone approximately 800m before foreshore Photo 4: Side gully on Green Creek identified in trigger event of 1989 landslide. Note relatively little accumulation of material and high trim line. Community of Quatsino 32 November 2014

33 MICHAEL CULLEN GEOTECHNICAL LTD Photo 5: Buildings in vicinity of Green Creek. Photo 6: Buildings in vicinity of Aweisha Creek Community of Quatsino 33 November 2014

34 MICHAEL CULLEN GEOTECHNICAL LTD Photo 7: Headpond above road on Aweisha Creek. Culverts identified with arrows Photo 8: Logged upper reaches of Aweisha Creek. Old slide tracks marked in red is one of suspected 1989 events. Community of Quatsino 34 November 2014

35 MICHAEL CULLEN GEOTECHNICAL LTD Appendix 1: Excerpts from Geomorphology of Vancouver Island Bedrock Geology, excerpt from Geomorphology of Vancouver Island: 2005, by R Guthrie and C Penner, British Columbia Ministry of Environment. Community of Quatsino 35 November 2014

36 MICHAEL CULLEN GEOTECHNICAL LTD Surficial Geology, excerpt from Geomorphology of Vancouver Island: 2005, by R Guthrie and C Penner, British Columbia Ministry of Environment. Community of Quatsino 36 November 2014

37 MICHAEL CULLEN GEOTECHNICAL LTD Mass Wasting Potential excerpt from 1:250,000 map Geomorphology of Vancouver Island, 2005, by R Guthrie and C Penner, British Columbia Ministry of Environment. Community of Quatsino 37 November 2014

38 MICHAEL CULLEN GEOTECHNICAL LTD Gully Process Potential, excerpt from 1:250,000 map Geomorphology of Vancouver Island, 2005, produced by R Guthrie and C Penner, British Columbia Ministry of Environment. Community of Quatsino 38 November 2014

39 MICHAEL CULLEN GEOTECHNICAL LTD Appendix 2: Seismic Hazard Community of Quatsino 39 November 2014